Вот несколько возможных решений:
Решение 1:
$this->db->select('*');
$this->db->from('subjects');
$this->db->where('strand', 'GAS');
$where = '(yrlvl = "11" or yrlvl = "1112")'; // make a OR statement by passing values here
$this->db->where($where);
$this->db->order_by('id', 'DESC');
$query = $this->db->get();
return $query->result_array();
Решение 2:
$this->db->select('*');
$this->db->from('subjects');
$this->db->where('strand', 'GAS');
$where = array('11', '1112'); // make an array of values for OR condition
$this->db->where_in('yrlvl', $where);
$this->db->order_by('id', 'DESC');
$query = $this->db->get();
return $query->result_array();